The U.S. Pat. No. B1 6,609,988 illustrates an asymmetric-damping belt tensioner comprising: a cup-shaped body defining a supporting pin; a tensioning arm connected to the supporting pin so that it can rotate; a pulley mounted on the tensioning arm so that it can rotate; and a helical thrust spring arranged concentrically about the supporting pin within the cup-shaped body. The known belt tensioner further comprises an annular friction element actuated by the thrust spring and driven by the tensioning arm via a multiplicity of stop elements carried by the tensioning arm and eccentric with respect to an axis of rotation of the tensioning arm.
In particular, the thrust spring comprises a first end portion rigidly connected to the cup-shaped body and a second end portion bent in the direction of the supporting pin and connected to the friction element in two contrast points so as to exert a torque on the friction element itself and pre-load the friction element against an internal cylindrical wall of the cup-shaped body.
Furthermore, the thrust spring exerts a pressure in the axial direction against the friction element, holding it against the tensioning arm.
During operation, the thrust spring is simultaneously subjected to variable torsional loads due to the angular movement of the tensioning arm, an axial load necessary to maintain the friction element axially in position against the tensioning arm, and a bending moment, localized on the second end portion, for pre-loading the friction element against the cup-shaped body.
Consequently, the thrust spring must have a structure dedicated to the asymmetric-damping application. In addition, design of the spring is critical, and the tolerances that can be obtained in production and in assembly can jeopardize the reliability of the tensioner. The friction element is likewise complex and costly to produce.